Police stings around the country have failed to find anyone selling synthetic drugs, on the first day of banned sales.

The Psychoactive Substances Act, which came into force at midnight, forbids sale of a product unless an official regulator appointed by the Minister of Health has approved it.

Under the new legislation, manufacturers must prove their product has a low level of harm before it can be sold.

Waikato police have discovered stores still in possession of the products, but spokesman Andrew McAlley said: "It's not an offence to be in possession of it, it's an offence to sell it."

Police would do all they could to oppose the sale of synthetic drugs, as they were concerned about societal harm, he said.

Some retailers may apply for licenses to sell approved products, but dairies, convenience stores and service stations cannot.

Cosmic Corner is one retailer applying to sell the products, and said it hoped to receive confirmation from the Ministry of Health tomorrow.

Deputy Police Commissioner Mike Bush said police would work closely with district health boards to enforce the law.

"For the first few weeks we will take an educative approach, but after that there will be strict enforcement of the new legislation," Bush said.

"There has been significant and justified community concern over the impact these products have had throughout the country, particularly on vulnerable young people, and this legislation is a key means to respond to this harm."

He encouraged the public to alert police if they thought a retailer was stocking the product illegally.